Microphone



Patented Aug. 20, 1946 UNITE 'rem OFFICE MICROPHONE 11 Claims.

This invention relates to microphones and more particularly tomicrophones of the close speaking type, useful in noisy locations.

In locations such as factories, mills, and the like, where microphonesare utilized as integral parts of anv interphone system or for thepurpose of communicating over a public address system, it is highlydesirable that such microphones be substantially insensitive to planesound waves at relatively low frequencies while displaying sufficientsensitivity to spherical waves from a nearby source, such as voicewaves, to render a transmitted message intelligible. For that purpose ithas been proposed to utilize bidirectional pressure gradient microphoneof the ribbon type, but it has been found that such microphones are notentirely satisfactory.

The response of the usual ribbon type of pressure gradient microphonenear a point source of sound is such that the low frequency output isvery greatly exaggerated. Specifically, if such a microphone is placed/2 inch from a source of spherical sound waves, the pressure gradientmagnitude will increase below 2,000 cycles as the frequency is lowered,at the rate of 6 db. per octave above the value that would be obtainedfrom a plane wave. This results in the speech being extremely boomy, andis a very undesirable characteristic.

It is, accordingly, an object of this invention to provide a closespeaking microphone, the response characteristic of which to closespeech shall be better than that of microphones heretofore known.

Another object of the invention is to provide a pressure gradientmicrophone that shall be relatively insensitive to interference fromplane sound waves at frequencies lying within the useful voice range.

A further object of this invention is to provide a close speakingmicrophone that shall have substantially fiat response to sphericalsound waves from a nearby source.

The foregoing objects and other objects ancillary thereto are obtainedthrough utilization of one or more stiffness controlled piezoelectrictransducer in a very small microphone of the pressure gradient type.Through utilization of such element or elements for the purpose orinducing potentials responsive to sound pressures,

the frequency-response characteristic of the im proved microphone in aplane sound wave will rise 6 db. per octave with increase in frequency.When such a microphone is disposed close to a point source of sound, therising characteristic inseam March 11, 1942, Serial No. 434,158

of the element or elements will ofiset the falling characteristic ofpressure gradient microphones in general with the result that theresponse is substantially flat over the major portion of the speechfrequency range. Accordingly, the discrimination against noise of apressure gradient microphone comprising a piezoelectric crystal elementis two-fold. In the first place, there is ordinary discriminationresulting from the cosine pattern of the pressure gradient unit, andsecondarily, there is further discrimination, increasing as thefrequency decreases, due to the loss in sensitivity at decreasingfrequencies in a plane wave sound field such as is produced by a distantsound source.

The novel features of this invention are set forth with particularity inthe appended claims. The invention itself, however, both as to its 01'-ganization and its method of operation together with further objects andadvantages thereof, will be understood best from a consideration of thefollowing description of certain specific embodiments taken inconnection with the accompanying drawing in which:

Fig. 1 is a view in perspective of a preferred embodiment of theinvention, partly broken away.

Fig. 2 is an enlarged cross-sectional view of one of the piezoelectriccrystal element assemblies, known as a Sound Cell, of the general typeutilized in the device shown in Fig. 1.

Fig. 3 is a greatly enlarged and dimensionally exaggerated diagrammaticview, in end elevation, of the Sound Cells, utilized in the device shownin Fig. 1.

Fig. 4 is an enlarged and dimensionally dis torted view, partly inelevation and partly diagrammatic, exemplifying another embodiment oithe invention, and

Fig. 5 is an enlarged and dimensionally dis tOlted view, partly in sideelevation and partly diagrammatic, exemplifying another embodiment ofthe invention.

In all figures of the drawing, identical elements are similarlydesignated.

Referring now to Fig. 1 of the drawing, a preferred embodiment of theinvention includes two Sound Cells 1, I of the general type disclosed inthe United States Letters Patents to A. L. W. Williams, 2,126,436 and2,126,438, and in the United States Letters Patent to C. B. Sawyer,2.105910, suitably supported in spaced apart parallel relation andprovided with means such as a mouthpiece 3 for directing the voicebetween them when the device is utilized for close speaking. In Fig. 1it will be noted that the mouthpiece is spaced away from the forwardedges of the Sound Cells and that a foraminous housing 5 permits planewaves from distant sources to reach both of the cells simultaneously.

Each cell is preferably constructed as exemplified by Fig. 6 of theWilliams Patent 2,126,436 or by the several figures in the WilliamsPatent 2,126,438 and the Sawyer Patent 2,105,010.

Referring now to Fig. 2 of the drawing, each Sound Cell comprises tWocrystal-element units or of the multiplate type generally designated 1,'1. Each element includes two plate-like crystal elements 9 and II, andit is provided with a central electrode 13 and two surface electrodes 15and H. The crystal plates 8 and H are cut parallel to the plane of the band c axes of the mother crystal of Rochelle salt or the like, with thesides or edges of the plates disposed at to said I) and c axes.

The crystal units 1, l. are mounted in parallel spaced relation in aframe-like support designated in its entirety by the numeral 19. Thetwounits 7, l are spaced apart and supported in relation to each otherby blocks 2!, 2| around which the units flex when subjected to soundpressures. The two crystal units are held in position on the lugs byelastic cement, or the like, and by sheets 23, 23 of suitable materialsuch as thin paper, rubber, Koroseal, parchment, or the like, which arecemented to the faces of the crystal units and to the top and bottomfaces of the frame lil, respectively. Preferably, the sheets areimpervious to moisture, but if pervious, they may be given a coat ofwater-proof material after assembly. The opposed faces of the units areshielded against sound by the frame and the covering sheets.

Although this invention is and hereinafter will be described asutilizing Sound Cells comprising four crystal sections each, it is notto be inferred thereby that it is limited by such description. On thecontrary, each crystal unit may be replaced by a plurality of unitssuch, for example, as shown in Fig. 4 of the Williams Patent 2,126,436,provided the electrodes are suitably connected into the output circuitas hereinafter will be described in connection with Figures 3, 4 and 6of the drawing.

Referring now to Fig. 3 of the drawing, it will be noted that, accordingto this invention, the central electrode I3 of one multiplate element ofeach Sound Cell is connected to the surface electrodes l5, ll of theneighboring element in the same cell to form a parallel opposing circuitso that the Sound Cell has pressure gradient characteristics and thatthe Sound Cells are connected together in a parallel opposing circuit.Since each Sound Cell itself has pressure gradient characteristics and anumber of Sound Cells are connected together in opposition it will beseen that the microphone may be made relatively insensitive insofar asinterfering plane waves are concerned, the space between the multiplateelements of each cell and the space between the cells being sufiiclentlysmall so that the frequency of maximum sensitivity may be placed farabove the highest frequency of the interfering sounds. For example, thespace may be of the order of .3 inch, placing the maximum sensitivity toplane waves at 20,000 cycles, 60,000 cycles, etc. Inasmuch as closerspacing is .possible, it will be seen that the microphone may be madesubstantially dead insofar as interfering plane waves are concerned, thespacing between th Sound Cells" being the controlling factor.

When used as a close speaking microphone, the mouth is held closelyagainst the mouthpiece 3 and the voice is directed between the spacedapart Sound Cells. Accordingly, the two inner multiplate elements aresubjected to greater sound pressures than the two outer elements of eachcell with the result that alternating potentials appear across theoutput terminals. Thus the mouthpiece 3 directs the voice onto one ofthe two sound sensitive surfaces of each of the two pressure gradientSound Cells. The connections of the microphones are such that theoutputs of the two pressure gradient microphones add for sounds directedby the mouthpiece. Interfering sounds originating at a distance actsubstantially equally on the two sound sensitive surfaces. Since thesound sensitive surfaces of each Sound Cell have, so to speak, oppositepolarities, the outputs of the cells are small for interfering soundsand these small outputs furthermore tend to cancel each other by virtueof the opposite connection of the "Sound Cells.

The instantaneous response of the assembl to a plane wave simultaneouslyreaching both Sound Cells is indicated by the plus and minus signs inFig. 3 of the drawing. When such waves arrive from a directionsubstantially parallel to the median plane of the cells, such responseis substantially zero.

It also lies within the scope of this invention to omit one of the soundcells, as indicated at a: in Fig. 3, and to direct the voice against theface of the remaining sound cell instead of between the two cells, asindicated by the dotted arrow in Fig. 3.

Furthermore, according to our invention we may connect the several soundcells in series-opposing balanced relation for substantiallysimultaneous actuation by sounds from distant sources, the entireassembly, as exemplified by Fig. 4 of the drawing then being comparableto a single pressure gradient microphone. In this case the pressuregradient assembly has, with re spect to the output terminals, two soundsensitive surfaces of one polarity (the surfaces of one sound cell) andtwo sound sensitive surfaces of the opposite polarity (the surfaces ofthe other sound cell). In such event, the output, as indicated by theplus signs at the terminals, is zero when sounds arrive from a sourcelying in a plane midway between the two inner Bimorphs and parallelthereto. For close speaking, the

, voice is directed against one of the end crystal element assemblieswhich induces sufficient potential to disturb the balanced conditionreferred to above. The lowest frequency to which the assembly hasmaximum response to plane waves is determined by the space between thesound cells.

Further, and still within the scope of this invention, twonon-directional microphones of the type disclosed in the copendingapplication of Alfred L. W. Williams and Dean R. Christian, Serial No.429,896, filed February 7, 1942, and assigned to the Brush DevelopmentCompany, may be utilized. Such modification is exemplified by Fig. 5 ofthe drawing and comprises two microphones 29, 29 disposed in spacedapart relation with the axes of the diaphragms thereinsubstantiallyparallel. As disclosed in the copending application, but not illustratedherein, each microphone includes a Bimorph of the twister type and twoopposed diaphragms, the said diaphragms being so coupled to oppositecorners of the Bimorph element that the force 2' applied to the saidBimorph in response to sound pressure on the diaphragms is cumulative.Such being the case, the direction from which the sound arrives is notcontrolling and the microphone is substantially non-directional. Whentwo such microphones are utilized as shown in Fig. 5, the leadstherefrom are connected to the output terminals of the assembly inseries-oppos ing relation. Accordingly, the assembly is insensitive tosound waves that reach all four diaphragms simultaneously but behaves asa pressure gradient microphone with respect to plane waves arriving fromsuch an angle that there is a lag between reception by one microphoneand reception by the other. Thus the diaphragms of one unit 29 of thpressure gradient assembly may be considered as the sound sensitivesurfaces of one polarity and the diaphragms of the other unit 28 may beconsidered as the sound sensitive surfaces of the opposite polarity.

For close speaking, the voice is confined .to one microphone byutilization of a mouthpiece, 3!, or the like, and in that event thepotentials developed by the said microphone in response thereto appearacross the output terminals.

Although certain modifications of this invention have been illustratedand described in detail, many others will be apparent to those skilledin the art. The invention, therefore, is not to be limited exceptinsofar as is necessitated by the prior art and by the spirit of thefollowing claims.

What is claimed is:

1. In a close speaking microphone assembly: at least two pressuregradient microphones each having a sound sensitive element so placedwith respect to each other that their planes of zero response are spacedapart, means for electrically connecting the said two microphonestogether with such polarity that their outputs tend to i cancel when theassembly is subjected to a plane low frequency wave, means associatedwith said two microphones for directing between the said two microphonessound waves which originate near-by whereby the outputs from the saidtwo microphones are added for said sound Waves which originate near-by.

2. The invention as set forth in claim 1 further characterized in this:that the said spaced apart planes of zero response are parallel to eachother.

3. The invention as set forth in claim 1 further characterized in this:that each of the said microphone units is stiffness controlled.

4. The invention as set forth in claim 1 further characterized in this:that each of the said microphone units is stiifness controlled and theoutput of each unit is approximately proportional to the displacement ofits sound sensitive element.

5. The invention as set forth in claim 1 further characterized in this:that each of th said microphone units includes piezoelectric generatingmeans.

6. The invention as set forth in claim 1 further characterized in this:that each of said pressure gradient microphones comprises a multiplateflexing element of Rochelle salt piezoelectric material having [theplanes of its major faces parallel to its said plane of zero response.

7. A microphone for close speaking comprising an assembly of two similarpressure gradient microphone units each of which comprises a soundsensitive element and a pair of electrical terminals, means forsupporting said units with their sound sensitive elements in spacedapart face-to-face relationship, circuit means between the electricalterminals of said two microphone units connecting the said two unitstogether with such polarity that when the microphone assembly issubjected to a plane sound wave of low frequency the outputs of the twounits tend to cancel and when sound originates close to and is directedinto the space between said units a sound sensitive element of each ofthe two units is actuated to develop an output which is added to theoutput of the other, and terminals for said microphone assemblyconnected to said circuit means and between which said additive outputis available.

8. The invention as set forth in claim '7 further characterized in this:that each of the said microphone units is stiffness controlled.

9. The invention as set forth in claim '7 further characterized in this:that each of the said microphone units is stiffness controlled and theoutput of each unit is approximately proportional to the displacement ofits sound sensitive element.

10. The invention as set forth in claim 7 further characterized in this:that each of the said microphone units includes piezoelectric generatingmeans.

11. The invention as set forth in claim 7 further characterized in this:that each of the said pressure gradient microphones comprises amultiplate flexing element of Rochelle salt piezoelectric materialhaving the planes of its major face parallel to the plane of the majorface of the multiplate flexing element spaced from it.

ALFRED L. W. WILLIAMS. ALFRED P. DANK.

